The present invention is related to shafts supported for motion by bushings; more particularly, to a shaft radially supported by a bushing for axial motion therethrough; and most particularly, to a shaft which is radially non-compliant to a bushing over a first portion of its length and which is radially compliant over a second portion thereof. A shaft in accordance with the invention is especially useful in centering a pintle of a pintle-type valve over most of the open portion of the pintle stroke while also permitting a valve head disposed on the pintle shaft to seek the corresponding valve seat in closing without constraints from axial misalignment of the shaft bushing.
Bushings are well known for use in guiding shafts during axial and/or rotary motion. Typically, a shaft is borne relatively snugly in a bushing to minimize radial runout of the shaft during its axial stroke or rotation. As such, the bushing is the centering element for the shaft.
In pintle-type valves, also known in the art as poppet valves, a pintle shaft supportive of a valve head is oscillatable axially to cause the head to be engaged with, and disengaged from, a valve seat to prohibit and permit, respectively, flow of material through the valve. In some valves, the pintle shaft extends through and is supported by a single main bearing or bushing in a wall of the valve housing opposite to the seat. In all valve-open positions, the pintle shaft and valve head, therefore, are cantilevered from the main bushing. If flow of material through the valve is especially turbulent or high velocity, side-loading forces on the pintle may cause it to vibrate, which can accelerate wear of the bushing, and may even permanently distort the pintle, leading to valve closing failure.
In such high-demand applications, it is known to provide a secondary shaft bushing for additional radial support of the pintle shaft. Such a secondary bushing may be provided either along the pintle shaft at a location above the extreme travel of the valve head, or along a shaft extension beyond the valve head.
A secondary support, however, presents a serious dilemma for manufacture and use of such a valve. On the one hand, for proper actuation, it is highly desirable that the valve head be free to mate with the valve seat; thus the shaft desirably has substantial radial runout in the bushing to prevent radial constraint of the shaft and head due to non-coaxial misalignment of the valve seat and secondary bushing in manufacturing or due to thermal distortions. On the other hand, to prevent vibration and deformation of the pintle shaft, it is highly desirable that the valve have minimal radial runout in the bushing.
It is a principal object of the present invention to provide means whereby a pintle shaft has minimal radial runout to prevent shaft vibration and also permits free seating of a valve head in a valve seat.
Briefly described, a pintle-type valve has a pintle shaft radially supported by a primary bushing in a first wall of the valve""s housing. The shaft extends through a chamber in the housing and supports a valve head near the distal end of the shaft for matably closing with a valve seat surrounding a port in a second wall of the housing. The shaft is provided with means for cooperating with a secondary bushing. The shaft is radially and non-compliantly supported by the secondary bushing through the major portion of the valve""s axial stroke. However, that portion of the shaft extension which is within the secondary bushing just as the valve closes is provided with a reduced diameter, such that the shaft extension then is not radially supported by the bushing and the valve head may freely self-center on the valve seat.